This invention relates to a method for chemically decontaminating water-cooled nuclear power reactor coolant systems. More specifically, this invention relates to a method for regenerating the reagents used for the chemical decontamination of the primary coolant systems of water cooled nuclear power reactors.
In nuclear reactors cooled by water, the primary materials of construction, that is, stainless steel, carbon steel, and Inconel, are continuously corroding at an extremely low rate to form corrosion products such as Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4. A percentage of these corrosion products are sloughed or leached from the corroding surfaces and the majority are deposited on the surface of the fuel cladding in the reactor core. Here, the corrosion products become radioactive by bombardment with neutrons from the fuel. The corrosion products, which now contain radioactive isotopes, are carried from the core by the circulating coolant and are redeposited on other surfaces of the cooling system in areas where they can expose workers in the power plant to radiation.
The presence of a radiation field around out-of-reactor equipment complicates maintenance and repair due to the need to minimize the exposure of the workers to the radiation. It is therefore desirable to reduce this radiation exposure to levels that are as low as reasonably achievable. One obvious method of accomplishing this reduction is to reduce the source of the exposure. Periodic decontamination of the primary coolant system of nuclear reactors appears to offer a practicable approach to control of the radiation exposure levels of personnel.
A method for decontaminating nuclear reactor coolant systems using a dilute chemical decontamination concept was developed by Atomic Energy of Canada, Ltd. The process known as CAN-DECON, is described in J. Br Nucl. Energy Soc., 1977, 16 Jan., No. 1, pages 53-61. As explained therein, a proprietary mixture of organic acids is added to the reactor coolant to form a 0.1% solution, and this solution is circulated throughout the reactor. The acids dissolve the oxide films and embedded radionuclides from the metal surfaces of the cooling system. The chelated metals are then transported by the circulating coolant to cation exchange resins in the purification system where the metals are removed, the organic acids regenerated, and the solution recirculated for further decontamination. When decontamination is complete, the coolant is passed through a mixed bed of cation- and anion-exchange resins to remove the reagents and any remaining dissolved metals from the coolant. Any solid material remaining in the coolant is removed by filters.
There are a number of advantages attendent to the use of this type of system. For example, since the coolant is used as the solvent for the decontamination reagents, the system does not need to be drained and the fuel can be decontaminated simultaneously. Since only very low concentrations of decontaminants are added to and removed from the coolant, corrosion of the coolant system is slight. The decontamination process can be continued as long as activity is still being removed since the organic acid reagents are being continuously regenerated. All wastes are concentrated on ion-exchange resins, which simplifies disposal. Also, no large storage tanks are required.
While this particular process works very well in heavy water reactors, it is not directly applicable to boiling water reactors (BWR) because the quantities of corrosion products (metal oxides) and radionuclides present on the fuel and out-of-core surfaces of BWR have been estimated to be as much as 100 times greater than the quantities that must be removed during heavy-water cooling system decontaminations.
In a dilute-chemical decontamination process, the concentrations of dissolved (complexed) metal ions (primarily Fe.sup.+3 and Fe.sup.+2) and radioactive ions (primarily Co.sup.+2) increase as the corrosion-product oxides are dissolved. When the concentration of dissolved metal ions approaches the complexing capacity of the organic acids, the dissolution rate decreases and the decontamination process becomes very inefficient and is, therefore, effectively terminated. Furthermore, if the concentrations of the metal-ion complexes exceed their solubility limits, precipitation of these radioactive compounds can occur; these precipitates can settle in dead-legs and low-flow regions, creating future operational problems.
It is therefore necessary to continuously remove the metallic ions as they are generated. This can be accomplished by continuously circulating some of the decontamination coolant through a regenerating ion exchange system that removes the metallic-ions without removing the organic chemicals.
In the prior art process, cation-exchange resin is utilized for the continuous regeneration of the dilute reagents which are thought to be a mixture of oxalic acid, citric acid, and ethylenediaminetetraacetic acid (EDTA). The regeneration process works adequately for the removal of the divalent ions (such as Fe.sup.+2 and Co.sup.+2) from the oxalate and citrate complexes. THis occurs because the divalent-ion complexes are so weak that chemical equilibria for the divalent ions favors the cation-exchange resin over the organic complex. However, the Fe.sup.+3 complexes with oxalate and citrate are considerably stronger, so that only a small fraction of the Fe.sup.+3 ion is removed by the cation-exchange resin. Furthermore, all of the EDTA-metal-ion complexes are sufficiently strong to prevent regeneration of EDTA with cation-exchange resin. In the heavy-water reactor decontaminations which have been performed, this inability to remove Fe.sup.+3 ions from their complexes has not been a significant problem due to the low quantities of corrosion products which accummulate in these reactors. However, in reactors where the quantities of corrosion products are considerably higher, a technique for removing the Fe.sup.+3 from the metal-ion complexes is needed to provide adequate complexing capacity without increasing the quantities of reagents which would lead to higher waste volumes and other problems.